Add gating tempest unit test job

This commit adds a gating python 2.7 unit test job for tempest. It
depends on change: I9efea4cfa42f770c34869c0b0931b0ed0b51c177

Change-Id: Id44ca50cd72a838d78c9bce87b96adb0f702e1a3
Merge "Add gating tempest unit test job"